1. Field of the Invention
The present invention relates to an image-forming apparatus which may be used in various devices such as, for example, laser printers, copy machines, fax machines, etc. which form an image on a reusable image carrier.
2. Description of the Prior Art
An image-forming apparatus comprises a photosensitive drum which is rotated in a predetermined direction. The drum is surrounded by a main charger, an exposure section of an exposure system, a developing unit, a transfer charger, a separation charger, a cleaner, and a de-electrifier, which are arranged successively in the rotating direction of the drum.
To form an image on a sheet of paper, first, the surface of the photosensitive drum is charged uniformly by the main charger. The charged surface is exposed by an exposure system (the exposure system may include an optical system such as in a copier, a laser-optical system such as in a laser printer, etc.) to form an electrostatic latent image on the surface of the photosensitive drum. After the latent image is formed on the drum, toner is then applied to the surface of the drum by the developing unit. As a result, the latent image becomes visible. In other words, it is developed into a toner image. Then, the toner image is transferred, by the transfer charger, to the surface of a paper sheet which is brought into contact with the surface of the drum. After the toner image is transferred to the paper, the paper sheet is separated from the drum by the separation charger.
Those toner particles remaining on the surface of the photosensitive drum, without having been transferred to the sheet surface, are removed by the cleaner. Thereafter, the electrostatic latent image on the surface of the drum is erased to complete one cycle of image formation.
Conventionally, residual toner particles on the surface of the photosensitive drum are scraped from the drum surface by a blade attached to the cleaner. The scraped toner is collected in the cleaner. Usually, the internal space of the cleaner is filled up with the toner after 2,000 to 3,000 sheets of paper are printed. As a result, the cleaner becomes unusable.
Image-forming apparatuses with the above described construction include ones which are designed so that the cleaner which is full of scraped toner particles can be discarded together along with the photosensitive drum when the drum is replaced. In these apparatuses, however, the "expendables" are expensive. In the case of frequently used apparatuses, such as printers, there is a significant "down time" during replacement of the cleaner and the drum. Therefore, such apparatuses are not preferred.
In some prior art image-forming apparatuses, a toner conveyor screw for toner recovery is provided within the cleaner. The screw conveys toner from the cleaner to a toner recovery box outside the cleaner, thereby recovering it. Because the recovery box is located inside the apparatus, it must be fairly small. After the image is formed on several thousands of sheets of paper, the box must be replaced with a new one. At the time of removal of the box, some of the toner may spill, thereby soiling the operator's hands or clothes or the floor.
Another problem with this conventional arrangement is that the blade of the cleaner is brought into contact with the surface of the photosensitive drum. Sometimes the surface of the drum is scratched by the blade.
An OPC (organic photoconductor) photosensitive drum is generally safe and harmless to use. However, due to its softness, the drum has only a very short life. If the photosensitive drum has a short diameter, in particular, it must rotate many times to accomplish recording on each paper sheet. Accordingly, one and the same portion of the drum must be used so frequently for printing each sheet that the life and the replacement cycle of the drum are inevitably short. It is not advisable, therefore, to use a slender photosensitive drum. Thus, the apparatus cannot be easily reduced in size.
In an effort to overcome these operational problems, image-forming apparatuses without cleaners have been developed. Such a device is described in U.S. Pat. No. 4,664,504, issued May 12, 1987, the teachings of which are incorporated herein by reference. In the device described in U.S. Pat. No. 4,664,504, the developing unit also has a cleaning function; that is, development and cleaning are effected simultaneously by means of the developing unit.
In the image-forming apparatuses of this type, however, defective cleaning is subject to occur; therefore, an image formed by the process corresponding to the preceding revolution of the photosensitive drum appears in a present image on a paper sheet.